The present invention relates in general to machining centers having a plurality of tools for successively machining a workpiece. More specifically, the invention provides an arrangement for automatically taking into account different lengths of successively used tools so as to relieve the operator of extensive "adjustment" each time one tool is exchanged for another.
Usually, in order to make the machining center perform the required machining, it is necessary to define a machining program. In defining the machining program, there is no means for knowing at what position a workpiece to be machined is positioned relative to the origin of mechanical coordinate axes (referred to as "MZP" (mechanical zero point), hereinafter). Therefore, an embodiment coordinate system is assumed for the purpose of defining the program. When machining is carried out, the mechanical coordinate values Xo,Yo and Zo of the origin of coordinate axes assumed in the program (referred to as "PZP" (program zero point), hereinunder), with respect to the mechanical zero point MZP in the direction of X and Y axes (parallel to the plane of the sheet in FIG. 1) and direction Z (perpendicular to the plane of the sheet in FIG. 1), are inputted to a machining center. In consequence, the coordinate values of the machining point in the program, expressed in terms of coordinate values with respect to the program zero point PZP, e.g. P1 (xl,yl,zl) into mechanical coordinate values (Xl,Yl,Zl) with respecct to the mechanical zero point MZP, and the tool is moved to the mechanical coordinate position (Xl,Yl,Zl) to effect the required machining.
Hitherto, the inputting of the mechanical coordinate values (Xo,Yo,Zo) of the program zero point PZP has been made by an operator by measuring the distance between the zero points PZP and MZP by a suitable method after mounting workpiece 3 on the machine. The measured distance is input via a key board or the like. According to this known method, it is necessary to measure the coordinate values (Xo,Yo,Zo) before machining, resulting in an impractically long preparation time and burden on the operator.
According to another method proposed hitherto, the coordinate values (Xo,Yo,Zo) are beforehand determined to eliminate the necessity of the operator measuring the distance. This method, however, requires the mounting of workpiece 3 on the machine at an impractically high precision, requiring much time and labor.
In a single-purposed machine which does not employ any tool exchange such as numerical control drilling machine, it is possible to conduct the machining by making use of the initial tool set position as the program zero point PZP. However, this method cannot apply to a machining center which employs various tools having a variety of lengths in the direction of an axis of the spindle of the machining center, i.e. in the direction of Z axis, because it is not possible to return the tool to the program zero point PZP if the reach of a particular tool is different from that of a preceding tool as a result of tool exchange. Therefore, the operator is obliged to measure and input the coordinate position (Xo,Yo,Zo) each time a tool is exchanged or to move the tool to the program zero point PZP by a manual instruction and to make a new registration of the zero point PZP, although actually such a registration is often made impossible due to the process of machining. Therefore, it has been impractical to adapt the control method used for single-purposed machines to the control of tool position in a multi-tool machining center.
This problem is serious particularly when attempting a small-amount of machining of a workpiece 3 having a comparatively simple configuration with the program zero point PZP scribed directly on the work. In such a case, machining efficiency is decreased undesirably due to the necessity for measuring and inputting the aforementioned coordinate values (Xo,Yo,Zo), which delays the operator in starting machining.